Modulation formats such as QPSK (Quadrature Phase Shift Keying) and Multilevel QAM (Quadrature Amplitude Modulation) are adopted as a digital modulation scheme used in today's wireless communications. In these modulation formats, transition between symbols entails amplitude modulation. Accordingly, the amplitude of transmission signals modulated by these modulation formats changes with time. A transmission signal combined with a carrier signal is referred to as an input signal. Research and development on amplifiers amplifying an input signal when the input signal is transmitted is being conducted. Note that the input signal is sometimes referred to as an RF (Radio Frequency) signal.
Polar modulation is known as a scheme that amplifies an input signal with high efficiency so that the output signal intensity widely changes in a dynamic range. Known examples of polar modulation schemes are EER (Envelope Elimination and Restoration) and ET (Envelope Tracking). In the EER, first a transmission signal is split into a phase component and an amplitude component. The phase component that has a certain amplitude is input into an amplification unit. The amplification unit operates around the saturation point at which the maximum efficiency is achieved. On the other hand, the amplitude component is input into a power supply modulator, where the amplitude component is amplified. The output voltage from the power supply modulator is used as the power supply for the amplification unit. The amplification unit having such a configuration acts as a multiplier that combines the phase and amplitude components of a transmission signal.
In the ET scheme, on the other hand, the amplitude component of a transmission signal is amplified by a power supply modulator and the output voltage from the power supply modulator is used as the power supply for the amplification unit. Only a phase-modulated signal that has a certain amplitude is input into the amplification unit in the EER scheme whereas a transmission signal including both of the amplitude component and the phase component is input in the amplification unit in the ET scheme. The ET scheme is less efficient in amplification than the EER scheme. However, the ET scheme does not require high accuracy of timing of combining the amplitude and phase components in the amplification unit. Accordingly, the ET scheme can be implemented more easily than the EER scheme.
As a related technique, an ET amplifier is disclosed in Patent Literature 1.
An amplifier according to the related technique and voltage waveforms around a power supply modulator and an amplitude circuit will be described with reference to FIG. 8. The amplifier includes an amplification unit 100, a power supply modulator 300 and a load 1900. The amplification unit 100 includes a choke coil 101, an amplification circuit 102 and a matching circuit 103. A configuration in which an FET (field effect transistor) is used as the amplification circuit 102 will be described as an example.
The power supply modulator 300 amplifies an input signal and output a voltage. The choke coil 101 inhibits high-frequency components included in current input in the amplification unit 100 that has a carrier frequency from passing through the choke coil 101. The amplification circuit 102 allows current amplified in proportion to current input at the gate terminal on the basis of power supplied from the power supply modulator 300 to flow from the drain terminal to the source terminal. The matching circuit 103 performs impedance matching for the amplification circuit 102.
An operation of the amplifier according to the related technique will be described.
First, the power supply modulator 300 amplifies the voltage 3000 of an envelope signal extracted from a transmission signal and outputs an amplified voltage 3100. In the amplification circuit 102, on the other hand, current amplified in proportion to an input signal input into the gate terminal on the basis of power supplied from the power supply modulator 300 flows from the drain terminal to the source terminal. In the load 1900, a voltage proportional to the current flowing from the drain terminal to the source terminal is output. The voltage 3200 at the drain terminal is equal to the sum of a voltage 3400a having a frequency component nearly equal to that of the amplified voltage 3100 and a voltage 3400b having a carrier frequency component.
As shown in FIG. 8, the power supply modulator 300 amplifies the voltage 3000 of the envelope signal and outputs the amplified voltage 3100. If the value of the impedance of the choke coil 101 can be assumed to be negligible in the frequency band of the amplified voltage 3100, the amplified voltage 3100 and the voltage 3400a will have the same waveform. In other words, the voltage 3000 of the envelope signal is proportional to the voltage 3400a. FIGS. 9A and 9B show the relationship between the voltage 3000 of the envelope signal and the voltage 3400a. Ideally, the voltage 3000 of the envelope signal is proportional to the voltage 3400a as in FIG. 9A. However, a certain degree of deviation from the proportionality was expected since the gain of an operational amplifier or the like that constitutes the power supply modulator 300 is frequency dependent.